...my Park Avenue Ultra has a rather interesting quirk in fuel consumption.
If I do 65 on a flat, I get around 33-36MPG.
That drops down to around 25MPG between 70 and 80MPH.
Yet when I reach 85 to 90MPH, the fuel mileage rises back to around 29-32MPG.
65MPH is around 1700-1800RPM, 80MPH is around 2700RPM.
Same gear, slightly more throttle.
Any theories?

umm maybe when you do 85-90 you are on open road so you stay the speed which is way you get better fuel milage. Just for the record my bonnievile fall milage falls off the shelf at 90. By that I mean I get only 23mpg